For users of medication delivery devices it may be difficult to determine when effective contact has been established between an injection needle and the body. Also, when using a medication delivery device it may be very difficult for the user of the device to determine the exact position of the tip of the injection needle in the body—i.e. whether the injection needle by accident has entered a vein or entered muscular tissue.
WO 02/060325 discloses an apparatus for non-invasive determination of the allowable penetration depth of an injection needle when injecting into the subcutis layer of the human body. The apparatus comprises an elongated housing with an ultrasonic measuring head for transmitting an ultrasonic signal into the human body and receiving a reflected signal reflecting from the muscle membrane separating the muscular tissue from the subcutis tissue. The apparatus includes an electronic circuit for analysing the ultrasonic signals and calculating the possible allowable penetration depth of the injection needle in order to avoid penetration of the muscle membrane. The allowable penetration depth of the injection needle is instantly shown in a display integrated in the housing in the format as the allowable standard needle sizes recommended for that particular anatomical injection site. It is a disadvantage of the apparatus shown in WO 02/060325 that it needs to be removed and replaced with a medication delivery device before the actual dose of medicament can be injected into the body. This replacement introduces the risk that the medication delivery device is not positioned at exactly the same spot as the apparatus suggested in WO 02/060325. If the positioning of the two apparatuses is not exactly the same the medication delivery device may be positioned at a position where the allowable penetration depth differs from what has been measured with the apparatus according to WO 02/060325.
U.S. Pat. No. 2,763,935 discloses an early arrangement for measuring the thickness of outer fat layers on bodies of humans or swine. According to the arrangements suggested in U.S. Pat. No. 2,763,935 one or more electrodes are inserted into the fat layer whereby electrical parameters of the fat layer are measurable. U.S. Pat. No. 6,391,005 relates to similar but newer arrangement for measuring the penetration depth into an object.
WO 03/092487 relates to a doser comprising a syringe with an injection needle which extends beyond the doser itself. The doser comprises an engagement face in the vicinity of the injection needle so that the engagement face rests against the surface of the tissue into which the injection needle is inserted during expelling of a set dose of medication. Detector means are provided on the engagement face to sample signals on the skin of the patient. In WO 03/092487 the detector means involves a sensor for sensing heart beat signals. Thus, when heart beat signals are detectable the engagement face abuts the surface of the tissue and the injection needle is inserted in the patient. It is a disadvantage of the arrangement suggested in WO 03/092487 that additional and specific sensors are required in order to determine whether contact to with skin has been established.
FR 2 622 457 A discloses a microprocessor-controlled device for administering or sampling liquid products as well as micro currents. The device comprises a syringe fitted with an injection needle. An electronic micro circuit controls the operation of an electric motor which actuates a rack which ensures penetration and retraction of the injection needle. The micro circuit further controls an electric motor which drives a rack actuating a piston. The device suggested in FR 2 622 457 A is particularly intended for use within mesotherapy. In one embodiment suggested in FIG. 4 of FR 2 622 457 A an external electrode is electrically connected to the device via a cable. This external electrode is adapted to be held by the patient to receive an injection. By measuring the conductivity between the injection needle and the external electrode the position of injection is determinable. The device suggested in FR 2 622 457 A is a rather bulky device. In addition, the electrode used for determining the position of injection is an external electrode. By applying an external electrode the device suggested in FR 2 622 457 A becomes even more bulky and less user friendly.
It is an object of the present invention to provide a simpler arrangement for determining when contact between injection needle and the body of the patient has been established. By simpler is meant that no additional sensors are required.
It is a further object of the present invention to provide a method for determining whether the injection needle has entered for example epidermis, dermis or muscular tissue. Injection into for example muscular tissue should be avoided since a medicament, such as insulin, injected into muscular tissue is absorbed much faster compared to a medicament injected into for example subcutis tissue.
To comply with the above objects, the present invention suggests a method for measuring an electrical parameter, such as the impedance, of a closed electrical signal path including the body of the patient using the medication delivery device.
It is an advantage of the present invention that an insertion of an injection needle into the body of a patient can be detected by simply measuring an electrical parameter of the closed electrical signal path. No additional equipment, except some basic electronics, is required in order to perform this measurement.
Furthermore, it is an advantage of the present invention that changes in human tissue may be monitored as the injection needle enters the body. Thus, it is possible to distinguish whether the injection needle is in non-muscular tissue, such as epidermis, dermis or subcutis, only, or whether the injection needle has entered a region being dominated by muscular tissue.